Passive radio frequency identification (RFID) tags, as they themselves are not designed with any battery, operate relying on electromagnetic energy sent from a card reader. Due to their simple structure and economical practicality, passive RFID tags have been widely applied in the fields of logistics management, asset tracking and mobile healthcare.
When a passive RFID tag operates, it will absorb electromagnetic energy, sent from a card reader, from the surrounding. After absorbing the energy, the passive RFID tag rectifies part of the energy into DC power for powering internal circuits of the passive RFID tag; and the passive RFID tag further inputs the other part of the energy to an internal modulation/demodulator circuit which will demodulate an amplitude modulation signal carried in this energy and send the demodulated signal to a digital baseband portion of the passive RFID tag for processing.
As the distance between the passive RFID tag and the card reader varies, the electromagnetic energy absorbed by the passive RFID tag during operating from the surrounding varies too. When the passive RFID tag is too close to the card reader or the electromagnetic energy sent from the card reader is too high, the strength of a signal received by the passive RFID tag will also be high, so that the voltage sensed on the coil exceeds the voltage-withstanding limit of a transistor for the rectifier module in the chip. As a result, the transistor is damaged permanently, and the RFID tag no longer functions.
The passive RFID tag transmits data to the card reader in a load modulation manner, and the coil at the card reader side acquires the data upon detecting change in the impedance of the coil at the RFID tag side. When the passive RFID tag is too close to the card reader or the electromagnetic energy sent from the card reader is too high, a load modulation signal coupled from the RFID tag side is likely to result in saturation of the receiving end of the card reader, thus to fail the communication. Such failure is more likely to occur in an RTF (Reader Talk First) communication mode where the card reader sends a command first and then waits for a response from the RFID tag.
In order to solve the aforementioned problems of voltage-withstanding reliability and saturation of reception at the card reader, it is required to provide an amplitude limitation processing circuit in the interior of an RFID tag chip circuit, in order to ensure that the voltage across both ends of an antenna on the RFID tag is limited to a predetermined value. The implementation of amplitude limitation may be carried out by a method of leaking current from a rectifier branch to the ground, so that the voltage level output by a rectifier is controlled. The most ideal design requires that a current leakage path may be effectively cut off in the case of an extremely weak field, that is, the current is completely not leaked; and in the case of a gradually enhanced field, the current leakage path may control the starting point of current leakage and the amount of leakage current at any time, so as to achieve the purpose of dynamic adjustment.